This invention relates generally to a flat panel display device having a plurality of electron guns for providing electron beams to electron beam guides and particularly to an electron beam injection structure for such a display device.
U.S. Pat. No. 4,128,784 to C. H. Anderson describes a beam guide for use in a flat panel cathodoluminescent display device. The display device is composed of an evacuated envelope containing a plurality of internal support walls which divide the envelope into a plurality of parallel channels. Each channel contains a beam guide extending along one wall of the envelope. An electron gun structure emits electrons which are launched into the beam guides as electron beams. The beam guides include a pair of spaced parallel ladder type meshes extending along and spaced from the backwall of the envelope. The meshes contain a plurality of apertures arranged in columns extending longitudinally along the paths of the beams. Each longitudinal column of apertures constitutes a separate beam guide. The apertures also are arranged in rows laterally across the width of the guide meshes. One line of the visual display is generated by ejecting the electron beams out of the guides of every channel through the apertures in a single row.
Display devices of the type described in the Anderson patent can be much larger than conventional cathode ray tubes. For example, a display having a 125 cm diagonal is contemplated. Such a display would have a 75 cm vertical dimension and a 100 cm horizontal dimension. Each electron beam, therefore, must propagate the full 75 cm dimension of the display area, as well as an additional distance in the gun area where the beams are injected into the beam guide.
The path of each electron beam from the cathode to the display screen can be divided into three distinct regions. The first is an injection region where electrons are drawn from a thermionic cathode and introduced into the guiding structure as beams. The second region is the beam guide proper where, ideally, the electrons propagate through periodic focusing fields along the beam guide axis with little or no change in beam size. At an extraction point, which can be at any period of the main guide, the beams are deflected out of the beam guide through one of the rows of apertures into the third region. This region is an accelerating, focusing, and deflection region which shapes the beam size and establishes the location where the beams impact the display screen.
The overall operation of a display device is improved by maximizing the percentage of emitted electrons which enter trajectories that are stable in the propagation region. The distribution of position coordinates and velocity vectors of the entering electrons typically do not have the form which is most satisfactorily propagated by a periodic focusing structure, and strong focusing must be used to achieve a high injection efficiency. However, optimum optical performance of the display is achieved by operating the periodic focusing structure at the weakest possible focus strength, which minimizes the visibility of wobble induced beam landing errors at the screen. Weak focusing also results in lower magnification of the beam image projected to the screen and higher resolution is obtained. However, for fixed beam injection conditions, lowering the focus strength of the guide results in poorer beam confinement, a loss in injection efficiency, and lower overall picture contrast, because of scattered electrons. For these reasons the operating parameters and beam guide geometry are compromises between the competing requirements of maximized electron injection and optimum propagation performance.
Efforts have been made to overcome these difficulties by placing a launch region between the electron injection region and the beam propagating region. One such effort is described in U.S. Pat. No. 4,263,529 to W. W. Siekanowicz, et al. This patent describes a flat panel display including multiple beam channels each of which encloses guide meshes extending along the length of the channels. Each channel includes modulation electrodes and a cathode to provide modulated electron beams to the guide meshes. The guide meshes extend between the modulation electrodes, and the electron beams are propagated along the channels in the space between the meshes. A plurality of pairs of launch electrodes are arranged to span the beam guide meshes. The conditions under which electrons are launched into the space between the guide meshes can be selected by the application of various biasing potentials to the pairs of launch electrodes. Accordingly, the conditions under which electrons are launched into the propagation space can be selected substantially independently of the conditions required for operation of the cathode and modulation electrodes, and of the focus voltage on the beam guides.
U.S. Pat. No. 4,359,671 to R. A. Gange also describes a flat panel display device which is devided into a plurality of channels, each of which includes beam guides and a cathode. A plurality of electrode pairs is arranged between the cathode and the beam guides. The application of various combinations of biasing potentials to the electrode pairs permits focusing of the electron beams prior to their injection between the beam guides, and allows the use of higher potentials to attract electrons from the cathode to maximize extraction of electrons from the cathode without affecting the optimum focusing required for propagation along the channels.
The structures described in U.S. Pat. Nos. 4,263,529 and 4,359,671 constitute improvements over display devices which do not utilize launch regions. However, both structures require additional electrodes in the display device and thus increase the complexity and cost of fabricating the display. Both devices also have operational disadvantages. In the Siekanowicz display, an increase in the potential on a pair of launch electrodes simultaneously increases the focusing through the associated guide apertures in both principal directions transverse to the propagation axis. In the Gange display, optimum injection is achieved in one aspect of the beam, but no provision is available for adjusting focus in the plane of the guides. According to the criteria set forth, and consistent with the details of the electron injection as given below, what is needed is a beam guide having optimum propagation focusing and which permits independent focus tailoring of the injected electron beam in all directions, without the need for a special launch section composed of special electrodes. The present invention fulfills these needs.